In the assembly process for automobile bodies and frames currently is use, various pressed (i.e., stamped) work related to parts of the frame such as the axle housing or the side rails are produced in lots by an outside firm or subcontractor. This work includes the exterior body panels, such as the roof panel, the outer door panels and the side panels for the body, and the interior body panels such as the floor pan and the interior door panels. Once these panels are pressed, each type is loaded onto its own special pallet. A truck or some similar conveyance is used to transport the pallets into the plant where the body welding and assembly line is located. There they are stacked temporarily until such time as they are needed. One of these pallets is then transported to a specified location on the line by a forklift or by a self-propelled driverless truck guided either by remote control or by cable, as disclosed in Japanese Patent Publication (Kokai-Kohou) Heisei 1-172078. A worker or robot then removes the desired work from the pallet as needed for welding or assembly.
With the production system described above, pallets on which numerous pieces of the required work are loaded and delivered to desired locations on both sides of the welding and assembly line. To supply pallets to the vicinity of the line, one must ensure that there is enough room near the line to accommodate the pallets. In recent years, the multiple mixed flow line on which the same line can accommodate a number of different models of car, has come to be widely used. For this type of line, a number of pallets of work for each model of car must be available near the line. Since this requires the use of space which serves no purpose at a given moment, it is not desirable from the point of view of production efficiency.
Moreover, as press (i.e., stamping) equipment has advanced in recent years, individual pieces of work have been made larger so as to reduce the number of welding and assembly processes on the assembly line. As a result, the size of the pallets themselves has increased proportionately, which frequently causes them to stick out into the passageways parallel to the line. From the point of view of safety, this is not a suitable situation.
To solve these problems, a system was devised whereby multiple pathways were provided in the body panel supply line so that it would be possible to supply the type of panel needed selectively. This system was disclosed in Japanese Patent Publication (Kokai-Kohou) Heisei 2-358, corresponding to U.S. Pat. No. 4,981,252.
However, this technology was unable to solve the problem of dead space required in the vicinity of the assembly line as described above.